A separate chelating stage is the most effective method for removing the heavy metals from pulp. However, it is also a mere pretreatment for oxygen chemical stages, and it does not delignify or bleach the pulp. Thus separate chelating stages or acid washes in ECF bleaching would be idle stages in terms of the process. In addition, it is to be taken into account that these stages would require a separate bleaching tower with washers, and thus investments would be required at the mill. Another option would be to take the resources required by this stage from the actual bleaching or delignifying stage. If this were done, the conditions should be made harsher in other stages, whereupon the strength of the pulp might suffer.
WO application publication 95/27100 describes a process for a complex treatment of pulp in connection with the chlorine dioxide stage. The complexing agents used are ethylene diamine tetra-acetic acid (EDTA) and diethylene triamine penta-acetic acid (DTPA). The poor biodegradability of the complexing agents can, however, be regarded as a problem.